Nd—Fe—B magnets have been recently developed as the leading RE permanent magnets with the highest room temperature magnetic properties beneficial for the wide use. The experimental value of the energy product of sintered Nd—Fe—B reached 59.5MGOe about 93% of the theoretic value and the remanence reached about 96% of the theoretic value in 2006, which was attained through the conventional single-alloy powder metallurgy method. Total weight of the 2007 production of Nd—Fe—B sintered magnets probably reached 58000 metric tones.
However, Nd—Fe—B permanent magnet materials have extremely poorer thermal stability than conventional Sm—Co permanent magnets. The coercivity of the magnet with highest energy product is as low as 8.2 kOe. Thus, they have suffered from the problems that they cannot be assembled in automobiles or precious devices appliances, and that they cannot be used in high temperature environments.
In order to make Nd—Fe—B sintered magnets more useful in a wider variety of applications at high temperature, the higher coercivity is essential. Because of the consequence of this, deficiency is reflected in the temperature coefficient of Br (remnant magnetism) and Hci (intrinsic coercivity), especially in the latter.
To address these problems, numerous researches have been carried out to improve their operating temperature. The elements addition is an effective approach, two types of substituent elements (S1, S2), which replace the rare-earth element (S1=Dy, Tb) or the transition element sites (S2=Co, Ni, Cr) in the hard magnetic phase, and two types of dopant elements (M1, M2) are distinguished. Substituent elements mainly change the intrinsic properties, such as spontaneous magnetic polarization, Curie temperature, and magnetocrystalline anisotropy. Both types of dopant elements influence the microstructure in a different way. M1 (Al, Cu, Zn, Ga, Ge, Sn) form binary M1-Nd or ternary M1-Fe—Nd phases, M2 (Ti, Zr, V, Mo, Nb, W) form binary M2-B or ternary M2-Fe—B phases.
Those efforts can affect the properties by changing the intrinsic behaviour of the matrix phase or improving the microstructure or both. In many instances, some properties of the sintered characteristics of the ternary Nd—Fe—B system are commonly improved but by sacrificing other properties. The reason is that one or several of the intrinsic magnetic properties of the matrix phase are impaired when these elements are dissolved in the matrix phase.
Based on the argumentation, it is necessary to find a method (or alloy) for improving the thermal stability (Tc and Hci) without impairing the magnetic performance (Br (remnant magnetism) and (BH)max (magnetic energy production)).